# CCL: What is the meaning of minus value of Mulliken atomic spin
densities?

*From*: Marcel Swart <marcel.swart]=[icrea.es>
*Subject*: CCL: What is the meaning of minus value of Mulliken atomic
spin densities?
*Date*: Fri, 13 Aug 2010 15:54:15 +0200

Sent to CCL by: Marcel Swart [marcel.swart]=[icrea.es]
First, much depends on the spin polarization that you have given,
i.e. in case of a doublet, if this doublet spin density should be
in the alpha spins, or in the beta spins.
(10 alpha 9 beta, vs. 9 alpha 10 beta)
In principle, the sign of the spin densities should depend on
this choice.
Second, it depends if the program takes into account the electric
charge for an electron or not. I.e. again looking at the 10alpha-9beta
system, the total number of spin-polarized alpha-beta electrons is +1,
with a corresponding total charge of -1. I'm almost certain that most
programs report the -1 (charge) and not the number (+1), but I'm not
sure.
Third, Mulliken charges are known to be very basis set sensitive.
Anyone of these factors influences your system, and even more so
when you have a (transition) metal present.
Marcel
On Aug 13, 2010, at 1:48 PM, wangzc^iccas.ac.cn wrote:
> Dear all,
>
> Recently I calculated a structure with MINUS value of Mulliken atomic
spin densities,anyone can tell me how to understand that minus value?
>
> Thanks very much!
===================================
dr. Marcel Swart
ICREA Research Professor at
Institut de Química Computacional
Universitat de Girona
Parc Científic i Tecnològic
Edifici Jaume Casademont (despatx A-27)
Pic de Peguera 15
17003 Girona
Catalunya (Spain)
tel
+34-972-183240
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e-mail
marcel.swart!^!icrea.es
marcel.swart!^!udg.edu
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===================================